1. Field of the Invention
The present invention relates to a robot control device which can prevent misjudgment in judging if a robot has collided with an obstacle.
2. Description of the Related Art
Some conventional robot control devices are provided with a collision judging part for judging if a moving part of a robot has collided with an obstacle. A collision judging part of a conventional robot control device estimates a disturbance torque which is applied to a moving part of a robot based on a rotational speed and drive torque, etc., of a servo motor, and compares the estimated value of the disturbance torque with a predetermined threshold value to judge if there is any collision of the moving part. In relation to this, the method of collision detection in JP-A-H04-242406 calculates the disturbance torque due to contact and collision with an obstacle by subtracting a frictional torque and torque due to the effect of gravity from the disturbance torque which is estimated by an observer.
In this regard, it is known that the drive torque of a servo motor changes by an amount of the weight of a workpiece in accordance with whether the robot is gripping a workpiece. Therefore, in order to accurately estimate a disturbance torque, it is necessary to switch a numerical value of a parameter relating to the weight of the workpiece among the various parameters which are used for estimate calculation, in accordance with whether the robot is gripping a workpiece. Furthermore, it should be noted that even if the robot grips a workpiece, the load of the workpiece may not be completely transmitted to the robot. This point will be explained with reference to the example of a robot for lifting up a workpiece placed on a table. In this case, even when the robot grips a workpiece, the load of the workpiece will not be transmitted to the robot at all or only part of the load of the workpiece will be transmitted through a hand to the robot as long as the workpiece is receiving the reaction force from the table. After that, at the instant when the workpiece is lifted up by the robot and leaves the top surface of the table, the entire load of the workpiece will be transmitted through the hand to the robot.
Therefore, in the conventional method of collision judgment, it is difficult to accurately estimate the disturbance which is applied to the robot during the period from when the robot grips the workpiece to when the workpiece leaves the table. Similarly, in the case where the robot is placing a workpiece on a table, it is difficult to accurately estimate the disturbance which is applied to the robot during the period from when the workpiece contacts the table to when the robot releases the workpiece. As explained above, in the conventional method of collision detection, it is difficult to accurately estimate the disturbance which is applied to the robot during the period when a state transition is underway from either one to the other of the state where the load of the workpiece is not transmitted to the robot and the state where the entire load of the workpiece is transmitted to the robot. For this reason, the conventional method is liable to make misjudgment in judging if the robot has collided with an obstacle during the period when the above state transition is underway.
In relation to this, there is known a method of collision judgment which judges if there is any collision, using a separate threshold value which is set for each region in the work space of the robot. For example, JP-A-H11-291190 proposes a method of collision judgment which uses one threshold value for judging welding state by welding electrodes in the region where a welding robot performs a welding process, and uses another threshold value for judging a collision of an arm or end effecter in other regions. Further, JP-A-2013-169609 proposes a method of collision judgment which uses a larger threshold value for collision judgment when an expected value of motor torque which is evaluated from a position, speed, acceleration, etc., of a servo motor exceeds a predetermined value, and JP-A-2001-353687 proposes a method of collision judgment which uses a threshold value for collision judgment, which is variable in accordance with an operating speed of the robot. However, even if these methods of collision judgment are used, it is not possible to prevent misjudgment during the period when the state transition is underway.
A robot control device is being sought which can prevent misjudgment in judging whether a robot has collided with an object even during the period when a state transition is underway from either one to the other of the state where the load of the workpiece is not transmitted to the robot and the state where the entire load of the workpiece is transmitted to the robot to the other.